1. Field of the Invention
This invention relates to brushless motors, and more particularly, to a brushless motor serving as a spindle motor for a disc drive, such as a CD-ROM (compact-disc read-only memory) drive, for spinning the disc during access operations.
2. Description of Related Art
Low-speed CD-ROM drives generally utilize a three-phase axial-gap brushless motor as the spindle motor for spinning the disc during access operations. Although this type of motor is low in performance and output torque, it is still widely used as the spindle motor for the CD-ROM drives since it does not produce the so-called cogging torque that seriously deteriorates the performance of the motor. Another drawback of this type of motor is that coils for the stator are difficult to wind and mount on the circuit boards.
High-speed CD-ROM drives, on the other hand, generally utilize a three-phase radial-gap brushless motor as the spindle motor for spinning the disc. This type of motor can spin the disc with high speed. However, one drawback to this type of motor is that its stator, whether of the 8-pole 9-slot type or the 12-pole 9-slot type, is constructed with salient type salientpoles which make winding the coils thereon difficult and laborious. The coil easily breaks down, is difficult to insulate, and has poor reliability.
Still one further drawback of the three-phase brushless motor is that it needs three Hall sensors for phase detection and six power transistors for its driving circuits. This large number of components causes the manufacturing cost of the motor to be very high.
Single-phase axial-winding and radial-gap motors are disclosed in U.S. Pat. Nos. 4,891,567; 4,899,075; 4,987,331; and 5,093,599, to name a few. In these motors, the coils are wound on a plastic bobbin in one axial direction only, which allows easy and less laborious winding of the coils to be, a high production yield, and ease of providing insulation thereto. Still, the component cost for these motors is low since only one Hall sensor for phase detection and four power transistors on the driving circuit are needed. The overall manufacturing cost of these motors is thus significantly lower than the above-mentioned three-phase brushless motors. However, there are still drawbacks to these single-phase motors, in that they will produce large torque ripples and cogging torque, such that the torque-versus-speed characteristic is very poor. This means that the output torque will be very low when operating at high speeds. For this reason, these single-phase motors are generally utilized in low-precision and fixed-speed applications, such as to drive a fan motor for cooling electric devices.
Most conventional CD-ROM drives utilize a three-phase axial-gap or radial-gap brushless motor as the spindle motor for spinning the disc. If single-phase brushless motors are employed to serve as the spindle motor for CD-ROM drives, they have the advantages of low manufacturing cost, high production yield, and ease of construction. But one prerequisite for this is elimination of the above-mentioned drawbacks of the conventional single-phase brushless motors.